Traditionally, a circuit is known that maintains the amplitude of a video signal. The circuit can maintain the amplitude of the sync signal, which is the difference between the pedestal level and the sync tip level of the video signal including the sync signal (such as a composite video signal and the Y signal of a Y/C separation signal). Such control is referred to as sync AGC.
More specifically, the ratio between the amplitude of the sync signal and the amplitude of the video signal representing white is a determined value as a standard. Accordingly, by controlling the amplitude of the sync signal at constant, the amplitude of the white portion of the video signal is also maintained at constant. As for images other than white, since the amplitude of the video signal is proportional to the brightness of the video signal and the white has the maximum amplitude, the amplitude of the video signal is uniquely determined, which corresponds to the brightness of the images.
The ratio between the amplitudes of the sync signal and the video signal representing white is set at 2:5 according to the ANSI/SMPTE170M standard (NTSC standard), and 3:7 according to another signal standard ITU-R BT.470 (PAL standard).
Other than this, a method is also known that maintains the peak value of the video signal at a fixed value. This control is referred to as peak AGC. The peak AGC also maintains the peak value of the video signal at constant, like sync AGC, by forming a negative feedback loop.
An AGC circuit disclosed in Japanese Patent Application Laid-open No. 10-164458 is an example that combines the sync AGC with the peak AGC, in which the sync AGC and the peak AGC operate simultaneously. Among input signals, there can be a video signal having amplitude exceeding the maximum amplitude of the video signal (amplitude of the white portion) determined from the amplitude of the sync signal according to the standard. As such, while the amplitude of the sync signal is less than the appropriate value, the amplitude of the video signal can exceed the maximum amplitude. In this case, a racing condition can occur between the operation to increase the gain by the sync AGC due to an insufficient sync signal and the operation to decrease the gain by the peak AGC due to an excess video signal.
In this condition, the control becomes unstable (or oscillates). Therefore, the time constant of the control loop of the peak AGC is set to shorter than that of the sync AGC, thereby preventing the control system from becoming unstable (see FIG. 5).
An AGC circuit disclosed in Japanese Patent Application Laid-open No. 2001-094826 employs a method categorized as peak AGC. This method provides three kinds of threshold values L1, L2 and L3 (L1>L2>L3) for the peak value of the video signal: when the peak value exceeds L1, its excess value is output as an error; when the peak value is greater than L2 and is less or equal to L1, zero is output as an error; when the peak value is greater than L3 and is less or equal to L2, a predetermined fixed error value is output; and when the peak value is less or equal to L3, zero is output as an error.
In this way, an excessive gain fluctuation is controlled such that the gain is reduced when the peak value of the video signal is in excess (greater than L1), the gain is maintained when it is at an appropriate level (greater than L2 and less or equal to L1), the gain is increased when the peak value is insufficient (greater than L3 and less or equal to L2), and the gain is maintained when the video signal is close to black (less or equal to L3) (see FIG. 4).
As for an apparatus that processes a given video signal to record or play back, it is desirable that once the optimum gain has been determined, the gain of the AGC is not varied unless the source of the video signal is switched.
As the sync AGC assumes that the ratio between the amplitude of the sync signal and the amplitude of the video has a predetermined value as described above, and has an advantage of not being dependent to the brightness of the images, it can easily achieve the desired characteristic.
However, as for actual input video signals, the ratio between the amplitudes of the sync signal and the video signal can often vary as signal distortions can occur due to a receiving condition of a radio wave or a recording condition onto a video tape.
Also, the sync signal is sometimes replaced during video editing, in which case the ratio between the amplitude of the images and the amplitude of the sync signal can go wrong.
As described above, a signal may be input with the ratio between the amplitudes of the video signal and the sync signal being deviated from the predetermined value. In many cases, the video signal may be input having an amplitude exceeding the maximum amplitude of the video signal determined from the amplitude of the sync signal according to the standard. Therefore, with a simple sync AGC, a problem can occur such that even if the gain is appropriate for the sync signal, the gain can be excessive for the video signal. When the gain of the AGC is excessive for the video signal, a large distortion occurs on the video signal, deteriorating the quality of the video. Disadvantages of the sync AGC are indicated in the foregoing prior arts.
The peak AGC, on the other hand, has an advantage in that the AGC can work even without the sync signal, and the video signal can be prevented from becoming excessive while the sync signal is appropriate as mentioned above.
For the peak AGC, however, even if the amplitude of the input video signal is at a peak value, it does not always mean the white level, nor ensure that it will be the maximum value of the video signal amplitude going forward. As such, the decision as to whether the gain to be increased or decreased has to rely on the brightness of the images around the detected peak, and hence, for a single signal source, the gain will always fluctuate depending on the brightness of the images. However, the gain fluctuation depending on the brightness of the images is undesirable for the apparatus that records or displays the video signal.
Japanese Patent Application Laid-open No. 2001-094826 proposes a method for improving the problem. The method has a certain range in the amplitude of the video signal, in which the amplitude of the video signal is determined to be appropriate and the gain is maintained. In addition, by suspending the gain control while in dark images, it is devised to prevent the gain from fluctuating depending on the brightness of the images. However, the range considered to be appropriate can vary from images to images. Therefore, it is configured to be able to input an appropriate level externally. However, it is unlikely that a single level can cover all possible images. Also, the problem of the gain fluctuation depending on the brightness of the video signal cannot be resolved completely.
Japanese Patent application laid-open No. 10-164458 proposes a method that combines the sync AGC and the peak AGC. The method, considering the case where both controls race each other, is directed to set the time constant of the peak AGC shorter than that of the sync AGC, thereby giving a priority of operation to the peak AGC and securing the stability of the control.
However, it only happens when the images are bright that priority is given to the peak AGC when the two controls are in race and hence the gain is restricted to prevent the peak amplitude from becoming too large. In this case, since the amplitude of the sync signal becomes smaller than a target value, the gain is increased by the normal sync AGC operation if the images become dark again. As such, the method cannot eliminate the possibility of the gain fluctuation depending on the brightness of the images with the same video signal source.
As described above, an AGC circuit is desired which can solve the foregoing problems and maintain the amplitude of the video signal at constant.